Processes for the production of polycrystalline silicon ingots are disclosed in the following documents: US Pat. No. 4,572,812 (Int. Cl. B29D 7/02, B22D 27/02 [1]), EP Pat. No. 1254861 (publ. 06.11.2002, (Int. CI. C01B 33/02 [2]), EP Pat. No. 1754806 (publ. 21.02.2007, (Int. Cl. C30B 11/00 [3]) and consist in charging a silicon raw material into the melting chamber of a cooled crucible enveloped by an inductor, forming a melt surface, melting and pulling the multicrystalline silicon ingot. None of the processes, however, describes melting conditions and ingot pulling conditions, which provide for sustained conditions for melt crystallization.
A process for the production of multicrystalline silicon ingots by the induction method, the process bearing closely on the invention, comprises charging a silicon raw material into the melting chamber of a cooled crucible enveloped by an inductor, forming a melt surface, melting while monitoring the output parameters of the inductor feed, and pulling the multicrystalline silicon ingot under controlled cooling conditions (EP Pat. No. 1930483, Int. Cl. C30B 35/00, C30B 29/06, C01B 33/02, publ. 22.02.2007, [4]). In the prior art process, melting is controlled by monitoring the output power of the inductor feed, wherein the measured frequency of an inverter is compared with the preset frequency thereof, and the output power of the heating means feed is simultaneously monitored, wherein the measured temperature on the ingot surface is compared with the preset temperature on the ingot surface.
Under such conditions, however, the crystallization of silicon in the ingot is unstable, because a constant changing of the output power of the inductor feed in the prior art process leads to a constant change in the rate of ingot crystallization to thereby unfavorably affect its quality.
Also, according to the prior art process, an increase in the depth of the melt requires a decrease in the output power of the inductor feed. In case of an increase in the depth of the melt by raising the melt surface, the operating frequency is increased and the output power of the inductor feed is decreased. On the one hand, these dependencies result in an increase in the rate of melt crystallization and, on the other, in a decrease in the rate of melting the charged raw material, and it can result in a complete filling up of the melt surface with the raw material and its sticking to the walls of the cooled crucible. In consequence, the pulling of the ingot will be forced to stop to melt down the raw material bridging the crucible, the regular melting process disrupted, the rate of melting slowed, and the production efficiency reduced.
The present invention is aimed at an improvement in the process for the production of multicrystalline silicon ingots by the induction method, wherein silicon crystallization would become stable, ingot quality higher, and production efficiency increased due to suggested process steps.